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Cite this:Mater. Horiz., 2020,

7, 143

Selective deposition of silver and copper filmsby condensation coefficient modulation†

Silvia Varagnolo, a Jaemin Lee, a Houari Amari‡b and Ross A. Hatton *a

Whilst copper and silver are the conductors of choice for myriad

current and emerging applications, patterning these metals is a

slow and costly process. We report the remarkable finding that an

extremely thin (B10 nm) printed layer of specific organofluorine

compounds enables selective deposition of copper and silver

vapour, with metal condensing only where the organofluorine layer

is not. This unconventional approach is fast, inexpensive, avoids

metal waste and the use of harmful chemical etchants, and leaves

the metal surface uncontaminated. We have used this approach to

fabricate thin films of these metals with 6 million apertures cm�2

and grids of B1 lm lines, through to 10 cm diameter apertures. We

have also fabricated semi-transparent organic solar cells in which

the top silver electrode is patterned with a dense array of 2 lm

diameter apertures, which cannot be achieved by any other scalable

means directly on an organic electronic device.

Introduction

Silver (Ag) and copper (Cu) are the dominant current carryingelements in modern electronics and solar cells, and also themetals of choice for a diverse range of emerging applicationsincluding flexible transparent electrodes and as platforms forbiological and chemical sensors for point-of-use healthcare andenvironmental monitoring.1–7 When structured on the sub-wavelength scale the unique optical properties of these metalsalso enables them to trap and channel visible light in the form ofsurface plasmonic excitations,8 rendering them ideal as active-elements for a plethora of future optoelectronic applications9

and as a platform for the nascent field of plasmonic nano-chemistry.6 For all of these applications these metals are

patterned by printing from costly colloidal solutions of nano-particles followed by sintering to fuse the nanoparticlestogether,10 or by selective removal of metal by etching usingharmful chemicals,11 or by electrochemical deposition.12,13

Whilst the latter has the advantage that it enables selectivemetal deposition, it is an inherently chemical intensive and slowsolution based process.12,13 Additionally, using all of thesemethods contamination of the metal surface by organic residuesis inevitable, which modifies the work function in an uncontrolledway14 and impedes subsequent chemical derivatization of thesurface, which is limiting for frontier applications in sensors andorganic electronics.

Vacuum evaporation is a proven low cost method for thelarge area deposition of metal films and so is widely used inthe food packaging industry, as well as being ubiquitous inresearch laboratories. Due to the industrial importance of themetallisation of insulating substrates a great deal of effort hasbeen directed at improving adhesion between evaporated metal

a Department of Chemistry, University of Warwick, CV4 7AL, Coventry, UK.

E-mail: Ross.Hatton@warwick.ac.ukb Department of Physics, University of Warwick, CV4 7AL, Coventry, UK

† All data supporting this study are provided as supplementary informationaccompanying this paper. See DOI: 10.1039/c9mh00842j‡ Current address: Imaging Center at Liverpool (ICaL), School of Engineering &School of Physical Sciences, University of Liverpool, L69 3GQ, Liverpool, UK.

Received 31st May 2019,Accepted 12th July 2019

DOI: 10.1039/c9mh00842j

rsc.li/materials-horizons

New conceptsHere we report the proof-of-principle of a new concept in the selectivedeposition of copper and silver electrodes that completely avoids metalwaste and the use of harmful chemical etchants, and leaves the metalsurface uncontaminated, the latter being particularly important forfrontier applications in sensors and organic electronics. We show thatboth silver and copper vapour do not condense on extremely thin(B10 nm) printed layers of specific organofluorine compounds, so themetal is deposited only where the organofluorine layer is not. The beautyof this approach lies in its versatility and accessibility, since vacuumevaporation of metals is proven as a low cost metal deposition method bythe packaging industry, and the shape and dimensions of the featuresdeposited is limited only by the printing method used to deposit thepatterned organofluorine layer. Here we have used micro-contact printingto demonstrate proof-of-principle, although the approach is not limitedto this printing method. We demonstrate the power of this approach byfabricating semi-transparent organic solar cells in which the top silverelectrode is patterned with a 6 million 2 mm diameter apertures cm�2

which, to our knowledge, cannot be achieved by any other scalable meansdirectly on an organic electronic device.

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films and glass and polymer substrates.15,16 Studies of the veryearly stage nucleation of Ag and Cu on plastics, equivalent to ametal thickness of 1–2 atoms,17,18 have shown that whilst allmetal atoms are initially adsorbed at the surface not all remain,with the proportion remaining adsorbed quantified in terms ofthe sticking or condensation coefficient, C. Whilst a great dealof effort has been directed at maximising C, reports pertainingto how C can be intentionally minimised, or how substrateswith very low C might be exploited for selective deposition ofmetals are sparse19–23 and the ‘Holy Grail’ of selective depositionof Ag and Cu has remained elusive until now. In addition to theobvious technological advantage for manufacturing industry, theemergence of an accessible method for the selective depositionof Ag and Cu opens the door to the accelerated research anddevelopment of single use, disposable devices that would other-wise be uneconomically viable due to the high cost of patterningfilms of these metals over macroscopic areas.

In this work we report an unconventional, inexpensive andhighly effective approach to the selective deposition of Ag andCu, based on the use of very small quantities of cheap, lowtoxicity organofluorine compounds in the form of printedthin films. We have identified that Ag and Cu vapours do notcondense on thin films of highly fluorinated organic com-pounds, and demonstrate this for metal deposition equivalentto 85 nm – a thickness sufficient to completely block visiblelight. When Ag and Cu are evaporated onto printed organo-fluorine layers with a pattern, metal is only deposited where the

organofluorine layer is not, ensuring that metal is depositedonly where it is needed. Consequently, there is no metalremoval step, which avoids metal waste and eliminates theadverse environmental impact associated with the use ofchemical etchants, leaving a pristinely clean metal surface.The beauty of this method lies in its simplicity, since vacuumevaporation of metals is a widely available technique and theshape and dimensions of the features deposited is only limitedby the printing method used. The generality of this approach isdemonstrated using both polymeric and small molecule fluori-nated compounds and by applying this approach to glass,plastic and silicon substrates for both Ag and Cu. The versatilityis demonstrated by fabrication of metal films with features onthe micron scale through to a tenth of a meter, and semi-transparent organic solar cells in which the top transparentsilver electrode is patterned with a dense array of 2 mm diameterapertures per square cm.

Results & discussion

To demonstrate the power of this approach we have usedmicro-contact printing (mCP) to print arrays of 2.5 mm circlesof (1H,1H,2H,2H-perfluorooctyl)trichlorosilane (FTS) (Table S1,ESI†) with a density of B 6 million apertures cm�2 onto high Ctransparent substrates (Fig. 1 and Fig. S1, ESI†). mCP uses anelastomer stamp with elevated features that make intimate

Fig. 1 Selective deposition of Ag on various substrates using a micro-contact printed (perfluorooctyl)trichlorosilane (FTS) layer. In each case thesubstrate surface is modified with a high C layer of MoO3�x (10 nm prior to FTS printing to guarantee the formation of a compact Ag film where the FTSlayer is not deposited). (a–c) Scanning electron microscope (SEM) images of an 85 nm thick Ag film on MoO3�x/glass with 2.5 mm diameter circularapertures where FTS is printed. (d and e) Cross-sectional scanning transmission electron microscope (STEM) images of 50 nm thick Ag with 2.5 mmdiameter apertures on MoO3�x/silicon. (f) Atomic force microscope (AFM) topographic image of a MoO3�x/glass substrate with a micro-contact printedarray of FTS circles and a representative cross-section (g), along one row of features (h). AFM topographic image of an 85 nm thick Ag layer depositedonto a MoO3�x/glass substrate patterned with FTS and associated cross-section (i), along one row of holes. The scale bars correspond to 1 mm.

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conformal contact with a substrate for the selective printing ofthin films on surfaces, and can be scaled to large areas24 as wellas being easily implementable on a laboratory scale.25 Thewidely available fluorinated molecule FTS is identified asoffering an extremely low C for both Cu and Ag deposited bythermal evaporation and is amenable to mCP. The materialsused to impart a high C to the different substrates used in thisstudy were molybdenum oxide (MoO3�x) and polyethylenimine(PEI), which are deposited as very thin films by vacuumevaporation and from dilute solution respectively.

Fig. 1 and ESI† Fig. S2–S5 show representative scanningelectron microscope (SEM) and cross-sectional scanning trans-mission electron microscope (STEM) images of 15 nm, 50 nmand 85 nm thick Ag films thermally evaporated onto FTSprinted films, from which it is evident that the metal selectivelydeposits only in those areas where the substrate is notcovered with FTS. Spatially resolved energy dispersive X-rayspectroscopy (EDXS) spectra acquired on metallized and non-metallized areas for films with a nominal Ag thickness of 15 nmand 85 nm (Fig. 2) confirm this conclusion. Analysis of thecross-sectional STEM image (Fig. 1d and e) confirm that thethickness of the metallized areas is equal to that which con-denses on the quartz crystal microbalance. When the depositedAg thickness is increased to 85 nm (Fig. 1a–c, h, i, c, d andFig. S6, ESI†), the apertures are still largely free of Ag, whichdemonstrates the broad range of metal thickness over whichthis approach is applicable.

Comparison of the STEM images for Ag films of 15 and50 nm thickness (Fig. 1d, e and Fig. S2–S5, ESI†) reveal thatthere are a few isolated Ag nanoparticles in FTS printed regionswhich are larger when the metal thickness is increased. It isalso clear from the STEM images that where isolated Agnanoparticles are present in printed areas they are invariably

located at the interface between the MoO3�x layer and theprinted silane layer (Fig. 1d, e and Fig. S4, ESI†), whichindicates that nucleation occurs where Ag can diffuse throughthe FTS layer reaching the underlying substrate. AFM images ofthe micro-contact printed substrates before metal depositionshow that FTS printed regions have a mound shape with acentral peak height of several tens of nanometres that tapers tozero thickness at the edges (Fig. 1f and g). Where Ag nano-particles are present in FTS printed regions they are mostprevalent at the edges of the circular apertures where theprinted layer is thinnest (Fig. 1a, b and Fig. S6, ESI†). Addi-tionally, a common artefact of the mCP process is a crescentshaped trench at the outer edge of the printed circular area,where the FTS is locally thinner (indicated with an arrow inFig. 1f and g), which correlates with a crescent shaped distribu-tion of Ag nanoparticles after metal deposition (Fig. 1b andFig. S6, ESI†). Together these observations provide compellingevidence that Ag nanoparticles form where the FTS layer is thinenough for the Ag to diffuse through to the underlyingsubstrate. Diffusion of metals into polymer substrates is wellknown,26–28 particularly during the initial stage of polymermetallization where the system is far from equilibrium, andis a consequence of the very weak intermolecular interactionsand open surface structure of most polymer surfaces at themicroscopic level.

To determine the minimum thickness of the FTS layer neededto achieve selective Ag deposition, glass derivatized with themonochlorosilane analogue of FTS, 1H,1H,2H,2H-perfluorooctyl-dimethylchlorosilane (FMS) was prepared (Table S1, ESI†). SinceFMS has only one chlorosilane moiety polymerisation cannotoccur and so the chemisorbed layer is limited to one monolayer,which is equivalent to B 1 nm thickness. It is evident from Fig. S7(ESI†) that a monolayer of FMS is not sufficient to block Agdeposition, because the sample is an intense dark green colourtypical of a dense array for Ag particles. To test the generalityof this finding a monolayer of the thiol analogue of FTS,tridecafluoro-1-octanethiol, was formed on an ultra-smoothsemi-transparent Au film16 using a deposition method knownto result in a compact thiol monolayer on Au.29 Again, it is clearfrom ESI† Fig. S8 that a monolayer was not sufficient to blockAg condensation. EDXS analysis shows that the amount of Agdeposited onto substrates derivatised with a monolayers of FMSand the thiol analogue is the same as on the substrates withouta monolayer (Fig. S7 and S8, ESI†). Collectively these data areconsistent with Ag atoms being initially adsorbed at the surfaceand diffusing sub-surface before either being ejected back intothe vapour phase, or nucleating at the substrate surface. Whilstthe extent of this sub-surface diffusion inevitably depends on thecomposition and structure of the printed film (the latter depend-ing on the deposition method) and the metal deposition rate,30

we find that an FTS layer with a thickness of Z10 nm is neededto achieve selective deposition: Fig. S9 and Table S2 (ESI†).

To demonstrate another deposition method for the FTSlayer, 3.2 cm2 glass slides were coated with a 10–20 nm thick filmof FTS deposited by spin coating, followed by evaporation of Agequivalent to a nominal metal thickness of 15 nm. It is evident

Fig. 2 Comparison of Ag selective deposition for different metal thick-nesses. (a) A single aperture in a 15 nm thick Ag film on MoO3x/glass. (b)Spatially resolved energy dispersive X-ray spectroscopy (EDXS) analysis ofthe region inside and outside the aperture shown in (a). (c) A singleaperture in a 85 nm thick Ag film on MoO3�x/glass. (d) Spatially resolvedenergy dispersive X-ray spectroscopy (EDXS) analysis of the region insideand outside the aperture shown in (c). The scale bars correspond to 1 mm.

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from the EDXS analysis (Fig. S10 and Table S3, ESI†) and fromthe photographs in Tables S1 and S2 (ESI†) that there is nosignificant metal deposition over the entire FTS coated slide.Conversely, Ag does deposit onto films of comparable thicknessof the hydrocarbon analogue of FTS; octyltrichlorosilane (OTS)(Tables S1 and S2, ESI†), which shows that the fluorocarbonbackbone is the essential feature of the molecule for selectivedeposition, rather than the silane moiety. This conclusion isconfirmed by the demonstration of selective deposition usingthe polymeric organofluorine:poly(vinylidene fluoride-co-hexa-fluoropropylene) (PVDF-HFP), which has no silane functionality(Table S1, ESI†). To show that selective deposition can beachieved over large areas we have increased the circular aperturesize from the micron-scale to a tenth of a metre (Fig. 3).

For specific applications it may be necessary to remove theprinted organofluorine layer and metal nucleation layer fromthose areas not covered with metal in order to minimiseunnecessary parasitic light absorption, as shown in Fig. 3 fora 10 cm diameter aperture in a 50 nm thick Ag film. It is evidentfrom the transmittance spectra in Fig. 3 that in this casethe MoO3�x metal nucleation layer contributes substantiallyto parasitic absorption for wavelengths below 450 nm. Due tothe very low thickness of the organofluorine layer needed toachieve selective deposition, it is easily and quickly removed byrising with a solvent that dissolves it, or in the case of a cross-linked silane layer such as FTS, by rinsing using 0.2 M tetrabutyl-ammonium fluoride/tetrahydrofuran solution, to realise arrays ofapertures of well-defined depth (Fig. S11, ESI†). The MoO3�x metalnucleation layer can then be removed by briefly rinsing in water.

To deposit optically thin metal films with well-definedthickness we have used a relatively low metal deposition rate

of B1 Angstrom per second. Increasing the deposition rate bya factor of 5–6 times (and the deposited metal thicknessto 70 nm) resulted in more nanoparticle deposition, althoughonly at the edges of the printed area: Fig. S12 (ESI†). Away fromthe edges, where the FTS layer is thicker, apertures are essen-tially free of metal. Increasing the metal deposition rate requiresthe temperature of the metal source to be raised, which increasesthe mean kinetic energy of the incident metal atoms. It istherefore reasonable to expect that the propensity of the metalatoms to diffuse into the printed layer will be higher when themetal deposition rate is increased. Consequently, to accommo-date higher metal deposition rate the thickness of the printedorganofluorine layer must be increased.

We have found that this approach to the selective depositionof Ag is also applicable to the lighter group 11 metal; Cu(Fig. S13, S14 and Table S3, ESI†). Cu does not bind stronglyto MoO3�x and so forms isolated particles for a nominal metalthickness of 15 nm (Fig. S13a and b, ESI†). For this reason asolution processed PEI layer was used in place of MoO3�x toachieve compact Cu films on metallized areas (Fig. S13c–e,ESI†). This method can be translated to flexible transparentplastic polyethylene terephthalate (PET) substrates for both Cuand Ag, using MoO3�x as an adhesion layer for Ag and PEI forCu (Fig. 4 and Fig. S13 and S15, ESI†).

Transparent flexible electrodes

To demonstrate an important application, we have fabricatedtransparent electrodes on flexible substrates based on a grid ofAg lines B1 mm wide and 50 nm thick, suitable as transparentelectrodes in flexible optoelectronics (Fig. 4d and e). In thiscase the grid spacing is 18.5 mm which corresponds to 10%

Fig. 3 Characterization of a 10 cm diameter hole in a silver film. Trans-mittance spectra (referenced to air) of a 10 cm diameter hole in a 50 nmthick silver film fabricated by printing an FTS layer using a PDMS stamp onMoO3�x (15 nm)/glass substrate. The thin and thick dashed red lines arerespectively the highest and average transparency of that measured atseven different locations in the aperture. (a) Sample after metal evapora-tion. (b) Sample after washing with tetrabutylammonium fluoride/tetra-hydrofuran solution and water.

Fig. 4 Silver grids. (a–d) SEM images of Ag grids on a MoO3�x/polyethyleneterephthalate (PET) substrate, fabricated by thermal evaporating 40 nm Agonto a MoO3�x/PET substrate patterned with a micro-contact printed FTSlayer. (e) Photograph of a 3.2 cm2 transparent electrode fabricated bythermal evaporating 50 nm Ag onto a MoO3�x/PET substrate patternedwith a micro-contact printed FTS layer forming a grid as shown in panel (d).The scale bars correspond to 5 mm.

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metal coverage (Fig. 4d). These electrodes have an average abso-lute transparency across the wavelength range 400–1100 nm ofB78% although 12% of the incident light is attenuated by thePET substrate and MoO3�x adhesion layer due to reflection andabsorption (Fig. S16, ESI†). Of the light transmitted through thesubstrate, only 11% is attenuated by the metal grid, whichcorrelates closely with the Ag metal coverage. Brief rinsing withtetrabutylammonium fluoride/tetrahydrofuran followed by waterincreases the absolute transparency to 80% due to the removal ofthe FTS and MoO3�x layer together with any Ag nanoparticles atdefect sites in the FTS printed regions. The very small improve-ment in transparency, which is in large part due to removal of theMoO3�x adhesion layer, demonstrates the effectiveness of theselective deposition approach. In this case the sheet resistance ofthis electrode is B30 Ohm sq�1, although this can be reduced byincreasing the thickness of the Ag grid lines without increasingthe area of the electrode obscured by the metal.

Top-electrode for semi-transparent organic solar cells

To further demonstrate the power of this approach for applica-tions, we have fabricated semi-transparent organic photovoltaic

devices in which the top Ag electrode has a dense array ofB2 mm diameter apertures (Fig. 5) fabricated using the afore-mentioned method. To our knowledge there is no other scalablemeans of realising a metal electrode patterned with this densityof features directly on top of an organic optoelectronic device.

Conclusions

The power of the approach reported herein for the fabricationof patterned Ag and Cu films lies in its versatility and simpli-city, since vacuum evaporation of metals is a widely availabletechnique and the shape and dimensions of the featuresdeposited is only limited by the printing method used. In thisstudy we have used micro-contact printing and spin-coating ofthe organofluorine layer to demonstrate proof-of principle.However, for practical applications we see no insurmountableobstacle to using printing methods compatible with roll-to-rollprocessing for the organofluorine layer such as flexographicprinting or rotary screen printing. This, together with the factthat roll-to-roll vacuum evaporation of metal films is a longestablished industrial process for low cost metallization offlexible substrates, opens the door to high throughput produc-tion of patterned Ag and Cu films on insulating and conductingsubstrates. Additionally, since there is no metal removal step,there is no metal waste or use of toxic chemical etchants, whichare critically important advantages in terms of the materialsand environmental sustainability of the approach. For manyemerging applications it is also extremely useful that the metalsurface is uncontaminated by lithographic resist residue, sinceorganic residues modify the work function in an uncontrolledway and can impede subsequent chemical derivatization of thesurface, which is limiting for frontier applications in sensorsand organic electronics.

Whilst the factors affecting the process of spontaneousdesorption of metal atoms (and thus C) from soft surfaces arestill to be fully elucidated,19–22 the early stage nucleationstudies of Faupel et al.17,18 have shown that even when C isvery low there is no direct reflection of the metal atoms fromsoft surfaces and so all metal atoms are initially adsorbed. Thebody of evidence presented here is consistent with an extremelyweak interaction between Ag/Cu atoms and the organofluorinesurfaces in question, so Ag and Cu adatoms quickly desorbfrom the surface before metal nucleation occurs. We haveshown that condensation of these metals can be inhibited onthe scale of centimetres, so lateral diffusion of adsorbed metalatoms across the organofluorine surface is not considered to bean important process. Also, it is evident from ESI† Table S1 thatthere is no correlation between the surface energy of theorganofluorine layer and the extent of Ag deposition, sincethe surface energy of the PVDF-HFP film and OTS films arecomparable, but metal is only deposited on the latter. Conse-quently the surface energy of the substrate is only a usefulindicator of whether the metal that remains on the surfaceforms nanoparticles or compact thin film at low metal thickness.What is distinct about highly fluorinated organic molecules and

Fig. 5 Semi-transparent organic photovoltaic devices. (a and b) SEM imagesof the patterned Ag electrode after coating with a ZnO layer. (c) Schematic ofthe device architecture: glass/ITO/PEDOT:PSS/PCE-12:ITIC-m:PC70BM/ZnO/m-contact printed FTS/Ag (17 nm)/ZnO/PDMS. (d) Representative currentdensity–voltage characteristics for devices with the structure shown in (c).(e) Total transmittance (referenced to air) of the semitransparent devices withthe structure shown in (c). Inset, photograph of one device.

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polymers as compared to their hydrocarbon analogues, is theexceptional weakness of the intermolecular attractive forces(or cohesive energies),31 which stems from the low polarizabil-ity and high ionization potential of the carbon–fluorine bondcombined with the relatively large intermolecular separationthat results from steric repulsion between fluorine atoms.32

Consequently, fluorinated molecules exhibit exceptionally lowboiling points33 together with low surface energies compared totheir hydrocarbon analogues (OTS (27.1 � 0.5 mJ m�2) vs. FTS(13.2 � 0.9 mJ m�2) Table S1, ESI†) and increased chemicalstability. It seems likely that the combination of weak inter-molecular interactions together with a very weak metal–moleculeinteraction enables facile diffusion of Cu and Ag atoms into andout of the surface of highly fluorinated hydrocarbon film, suchthat the metal atom flux adsorbed at the surface is balancedby that of desorbing adatoms. Whilst computational studiesproviding further fundamental insight into the molecule–metalvapour interaction are underway, the findings reported hereinenable immediate practical implementation of this approach topatterning silver and copper thin films for numerous currentand emerging applications.

Funding

The authors would like to thank the United Kingdom Engineer-ing and Physical Sciences Research Council (EPSRC) for fund-ing (Grant number: EP/N009096/1).

Author contributions

SV performed all of the experimental work, except the STEMimaging. HA prepared the samples for TEM imaging andcollected the STEM images. SV and RAH designed the experi-ments, analysed the data and wrote the manuscript. RAHconceived the study and secured funding for the project.

Conflicts of interest

The work reported in this paper is disclosed in the followingnew UK patent application: ‘Selective deposition of metalliclayers’ The University of Warwick, filed 18th October 2018.Application No. 1817037.3.

Acknowledgements

The authors kindly thank Mark Crouch (Department of Engi-neering at the University of Warwick) for the use of the clean-room facility to produce the silicon masters and ProfessorMarin Alexe for his insightful advice.

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